1. Field of the Invention
The present invention relates to a dry toner for a developer, which develops latent electrostatic images in, for example, electrophotography, electrostatic recording, and electrostatic printing. More specifically, it relates to a toner for electrophotography which is used, for example, for copiers, laser printers, facsimiles for plain paper using a direct or indirect electrophotographic developing system. Further, the present invention is directed to a toner for electrophotography which is used for full-color copiers, full-color laser printers, and full-color plain paper facsimile machines using a direct or indirect electrophotographic multicolor developing system.
2. Description of the Related Art
In electrophotography, electrostatic recording and electrostatic printing, a developer is, for example, applied to an latent electrostatic image bearing member such as a photoconductor, so as to dispose the developer onto a latent electrostatic image formed on the latent electrostatic image bearing member in a developing step, the developer disposed on the image is transferred to a recording medium such as a recording paper in a transferring step, thereafter the transferred developer is fixed on the recording medium in a fixing step. Such developers used for developing the latent electrostatic image formed on the latent electrostatic image bearing member generally include two-component developers containing a carrier and a toner, and one-component developers such as magnetic toner and non-magnetic toners, which do not require a carrier.
Conventional dry toners for use in electrophotography, electrostatic recording or electrostatic printing are formed by melting and kneading a toner binder such as a styrenic resin or a polyester, a colorant, and other components, then pulverizing the kneaded substance.
Charging Properties
The toner is generally charged by friction. For example, it is charged as a result of contact friction with a carrier in a two-component developer, and it is charged as a result of contact friction with a feed roller for feeding the toner to a developing sleeve or with a layer thickness controlling blade for uniformizing the toner layer on the developing sleeve. To reproduce latent electrostatic images on an image bearing member such as a photoconductor exactly, charging properties of the toner are important. To yield satisfactory charging properties, a variety of attempts have been made on the types and incorporation processes of a charge control agent into a toner.
Such charge control agents play their roles on the surface of toner particles, and most of them are expensive. Accordingly, attempts have been made to arrange a small amount of a charge control agent on the surface of toner particles. Japanese Patent Application Laid-Open (JP-A) Nos. 63-104064, 05-119513, 09-127720, and 11-327199 disclose techniques in which a charge control agent is applied to the surface of toner particles to impart charging ability to a toner. However, the charging ability of the resulting toner is insufficient, the charge control agent tends to flake off from the surface, and toners having target charging ability cannot be provided even according to the production process disclosed therein. In particular, these patent publications never take an initial charging rate of the toner into consideration.
JP-A No. 63-244056 discloses a method in which a charge control agent is adhered and fixed on the surface of mother toner particles utilizing an impulse force generated at a gap between a rotor (i.e., a blade rotated at a high speed) and a stator (i.e., projections fixed on the inside wall of a vessel). However, since the inside wall has projections, turbulent flows tend to be formed, and thereby the particles may be excessively pulverized or partially melted, the charge control agent may be embedded in the surface of the mother toner particles or adhered to the surface unevenly. This is probably caused by unevenness in energy imparted to the particles. Japanese Patent (JP-B) No. 2962907 describes the relation between the amount of a charge control agent on the surface and that inside of the toner. However, this technique is still insufficient to improve the image-fixing properties of the toner.
Image-Fixing Properties
These dry toners are, after used for developing and transferred on a recording medium such as a sheet of paper, fixed on the sheet by heating and melting the toner using a heat roller. If a temperature of the heat roller is excessively high, in this procedure, “hot offset” occurs. Hot offset is the problem that the toner is excessively melted and adhered onto the heat roller. If a temperature of the heat roller is excessively low, on the other hand, a degree of melting the toner is insufficient, resulted in insufficient image fixing. Accordingly, there are demands in a toner having a higher temperature at which hot offset occurs (excellent hot offset resistance) and a low fixing temperature (excellent image-fixing properties at low temperatures), in view of energy conservation and miniaturization of apparatuses such as copiers. Toners also require a heat-resistant storability that suppresses blocking of toner when the toner is stored, and at a temperature of atmosphere inside the apparatus where the toner is accommodated. Especially, low melting viscosity of toner is essential in full-color copiers and full-color printers in order to yield high gloss and excellent color mixture of an image. As a consequence, polyester toner binders which melts sharply has been used in such a toner. However, this toner tends to cause hot offset. To prevent hot offset, in full-color apparatuses, silicone oil has conventionally been applied on the heat roller. Yet, in the method of applying silicone oil to the heat roller, the apparatuses need to equip an oil tank and an oil applier, therefore the apparatuses become more complex in their structures and large in their size. It also leads a deterioration of the heat roller, so maintenance is required at every certain term. Further, it is unavoidable that the oil is attached to recording media such as copier paper and films for OHP (over head projector), and especially with the films for OHP, the attached oil cases deterioration in color tone.
To prevent a toner fusion without applying an oil to a heat roller, wax is generally added to a toner. In this method, however, releasing effect is largely affected by a condition of dispersed wax within a toner binder. Wax does not exhibit its releasing ability if the wax is compatible with a toner binder. Wax exhibits its releasing ability and improves releasing ability of toner when the wax stays within a toner binder as incompatible domain particles. If a diameter of domain particles is excessively large, the resulting toner may not yield images with good quality. This is because a ratio of wax occurring in a surface portion of a toner with respect to other components of the toner increases with an increasing diameter thereof. The toner particles aggregate to impair fluidity of the toner. Moreover, filming occurs where the wax migrates to a carrier or a photoconductor during long-term use. Color reproducibility and clearness of an image are impaired in the case of color toners. On the contrary, if a diameter of the domain particles is excessively small, the wax is excessively finely dispersed so that sufficient releasing ability cannot be obtained. Although it is necessary to control a diameter of wax as mentioned above, an appropriate method thereof has not been found yet. For example, in the case of toners manufactured by pulverization, control of wax diameter largely relies upon shear force of mixing during melting and kneading procedures. Polyester resins conventionally used for a toner binder have a low viscosity, and sufficient shear force cannot be added thereto. It is very difficult to control distribution of wax and to obtain a suitable diameter especially for these toners. Another problem of pulverization is that more wax is exposed from a surface of toner, since a toner material article tends to broken at portions where the wax occur as a result of pulverization, and such broken portions of the wax constitute surfaces of the toner particles.
Particle Diameter and Shape
Although improvement of toners has been attempted by miniaturize a diameter of toner particle or narrowing particle diameter distribution of toner in order to obtain high quality images, uniform particle shape cannot be obtained by ordinary manufacturing methods of kneading and pulverization. Moreover, the toner is still pulverized so that excessively fine toner particles are generated, in a course of mixing with carrier in a developing member of the apparatus, or, by a contact stress between a development roller, and a toner applying roller, a layer thickness controlling blade, or a friction charging blade. These lead to deterioration of image quality. In addition, a superplasticizer embedded in the surface of toner also leads to deterioration of image quality. Further, fluidity of the toner particles is insufficient because of their shapes, and thus a large amount of the superplasticizer is required or a packing fraction of the toner into a toner vessel becomes low. These factors inhibit miniaturization of apparatuses.
A process for transferring, in which an image formed by a multicolor toner is transferred to a recording medium or a sheet of paper, becomes more and more complicated in order to form full-color images. When toners having non-uniform particle shapes such as pulverized toners are used in such a complicated transferring process, missing portions can be found in the transferred image or an amount of the toner consumption becomes large to cover the missing portions in the transferred image. This is due to the impaired transferring ability caused by non-uniform shapes of the toner particles.
Accordingly, a strong demand has arisen to yield high quality images which do not have any missing part and to reduce running cost by further improving transfer efficiency leading to a reduction in toner consumption. If transfer efficiency is remarkably excellent, a cleaner, which removes remained toner on a photoconductor or a transfer after transferring, can be omitted from an apparatus. Therefore, the apparatus can be miniaturized and low cost thereof can be achieved together with having a merit of reducing a waste toner. Hence, various methods for manufacturing a spherical toner have been suggested in order to overcome the defects caused by a non-uniformly shaped toner.
Conventional Attempts to Solve the Problems
Various investigations have been done to improve properties of toner. For example, a releasing agent (wax) having a low melting point, such as a polyolefin, is added to a toner in order to improve image-fixing properties at low temperatures and offset resistance.
JP-A Nos. 06-295093, 07-84401, and 09-258471 disclose toners that contain a wax having a specific endothermic peak determined by DSC (differential scanning calorimetry). However, the toners disclosed in the above patent publications still need to improve image-fixing properties at low temperatures, offset resistance and also developing properties.
JP-A Nos. 05-341577, 06-123999, 06-230600, 06-295093, and 06-324514 disclose candelilla wax, higher fatty acid wax, higher alcohol wax, vegetable naturally occurring wax (carnauba wax and rice wax), and montan ester wax as a releasing agent of toner. However, the toners disclosed in the above patent publications still need to improve developing properties (charging ability) and durability. If the releasing agent having a low softening point is added to a toner, fluidity of the toner is decreased hence developing properties or transferring ability is also decreased. Moreover, charging ability, durability and storability of the toner may be deteriorated thereby.
JP-A Nos. 11-258934, 11-258935, 04-299357, 04-337737, 06-208244, and 07-281478 disclose toners which contain two or more releasing agents in order to enlarge a fixing region (non offset region). However, the releasing agents are not dispersed sufficiently uniformly in these toners.
JP-A No. 08-166686 discloses a toner which contains polyester resin and two types of offset inhibitors having different acid values and softening points. However, the toner is still insufficient in developing properties.
JP-A Nos. 8-328293, and 10-161335 each disclose a toner that specifies a dispersion diameter of wax within the toner particle. However, the resulting toner may not exhibit sufficient releasing ability during fixing since a condition or positioning of the dispersed wax is not defined in the toner particle.
JP-A No. 2001-305782 discloses a toner in which spherical wax particles are fixed onto the surface of toner. However, the wax particles positioned on the surface of toner decreases fluidity of the toner and thus developing properties or transferring ability of the toner is also decreased. In addition, charging ability, durability, and storability of the toner may also be adversely affected.
JP-A No. 2002-6541 discloses a toner in which wax is included in the toner particle and the wax is located in a surface portion of the toner particle. However, the toner may be insufficient in all of offset resistance, storability, and durability.
Generally, a toner is manufactured by methods of kneading pulverization in which a thermoplastic resin is melted and mixed together with a colorant, and a releasing agent or a charge control agent may be further added according to necessity to form a mixture, and the mixture is pulverized and classified. Further, inorganic or organic fine particles may be added onto the surface of toner particle in order to improve fluidity or cleaning ability. In conventional methods of kneading pulverization, a shape and surface structure of toner particle are not uniform. Although depending on crushability of materials and conditions of pulverizing step, it is not easy to control a shape and surface structure of toner particle arbitrarily. In addition, a particle diameter distribution of a toner cannot be significantly further narrowed, due to limited classifying performance and an increased cost thereby. Regarding a pulverized toner, it is a great task to control an average particle diameter of toner particle to a small particle diameter, especially 6 μm or less, from the viewpoint of yield, productivity and cost.
Objects and Advantages
Accordingly, an object of the present invention is to provide a dry toner, which has improved image-fixing properties at low temperatures and offset resistance with low electric power consumption, forms a high quality toner image, and has an excellent long-term storability.
Another object of the present invention is to provide a dry toner, which has stable charging ability and can always yield a high-quality image with high resolution.
The present inventors has accomplished the present invention based on intensive investigations to develop a dry toner which does not require an application of oil to a heat roller, has excellent image-fixing properties at low temperatures, hot offset resistance, and heat-resistant storability, exhibits stable charging ability and can always form a high-quality image with high resolution. Especially, the investigations have been made for improving a toner which has excellent particle fluidity and transfer ability in the case of a toner having a small particle diameter.